The long term objectives of this research are to define the biochemical and molecular events that characterize the bacterial colonization of animal epithelial tissue. To accomplish this goal we have chosen as a model system the benign, symbiotic infection of the light-emitting organ of the squid Euprymna scolopes by the luminous bacterium Vibrio fischeri. This system provides a simple, experimentally accessible, paradigm for studying specific host-bacterial interactions. Our recent investigations have centered on the events characterizing the initiation, colonization, and persistence of the symbiotic infection in newly hatched juvenile squids using bacterial mutants and host morphogenic processes to manipulate and assay the complex succession of signaling and responses through which the host and bacterium communicate. Our specific aims are to: 1. Determine whether the ability to utilize cAMP is required for a successful V. fischeri colonization of host epithelium, and what the genetic basis for that requirement may be. 2. Discover V. fischeri colonization factors whose genes are regulated by autoinducer accumulation. 3. Examine whether the V. fischeri Cix-analog, Hvn is responsible for the cholera-toxin like effects that are induced in host epithelia during symbiotic colonization. 4. Determine the extent to which the host excretes cAMP into the light organ crypts in response to bacterial colonization. We believe that this system serves as a model of both benign and pathogenic infection and, perhaps of evolution of the virulence state in vibrios; that is, identification of bacterial and host determinants that potentiate light organ symbiosis may reveal convergences with known V. cholerae virulence factors, and promote the discovery of as yet undescribed ones. This work will also aid our understanding of the mechanisms by which benign colonizations of mollusk tissue serve as a reservoir for human pathogenic Vibrio species. The work outlined here will be among the first genetic and molecular studies of athe V. fischeri- E. scolopes symbiosis. It is expected that the identification of inducible genes in the bacterium will lead to a search for other components of signaling pathways that coordinate initiation, specificity and stable maintenance of a bacterial colonization of epithelial tissues.